Pathogenic utoantibodies often return following treatment in systemic lupus erythematosus (SLE), suggesting that long-lived plasma cells (PC) are not sensitive to such interventions. Therefore, their elimination is a key therapeutic goal in treatment. The goal of these studies is to validate P-selection glycoprotein ligand-1 (PSGL-1) as a novel marker for plasma cells in lupus, and as a therapeutic target in SLE. This work is based upon the observation that a subset of B cells differentiating into antibody secreting cells in germinal centers, the site of B cell memory and PC formation, express PSGL-1, with its expression highly upregulated on PC. The hypothesis to be tested is that PSGL-1 expression on antibody-secreting cells plays an important role in their trafficking to the bone marrow and splenic red pulp, niches for their survival. To address this hypothesis, three specific aims are planned. First, PSGL-1 expression and function on PC in normal mice following immunization will be assessed. Next, similar studies will be carried in lupus-prone mice, followed by blockade of PSGL1 interactions with its ligands, asking if PC trafficking to survival niches is altered, leading to diminished survival. Finally, PSGL- 1+CD38hi PC will be characterized in the peripheral blood of lupus patients, correlating their numbers with disease activity in an effort to support the idea that PSGL-1 on PC is an appropriate therapeutic target in SLE.